1. Field of the Invention
This invention relates to methods and apparatus for measuring the diameters of optical waveguide fibers and, in particular, to techniques for measuring such diameters with improved precision.
2. Description of the Prior Art
The precise measurement of the outside diameter of optical waveguide fibers is of central importance in both the manufacturing and quality control of such fibers. Among other things, diameter measurements are used to control the fiber drawing process and to select fiber suitable for commercial use.
U.S. Pat. Nos. 3,982,816 and 4,067,651 to Lawrence Watkins disclose an optical technique for measuring fiber diameter which is widely used in the industry. The basic components of the Watkins system are schematically illustrated in FIG. 1. As shown therein, optical waveguide fiber 13, whose cross-section has been greatly expanded for purposes of illustration, is transversely illuminated by light 15 of sufficient spatial coherence and monochromaticity to create a discernible interference pattern in the far field, that interference pattern being created by the superposition of light reflected from the fiber surface 17 and light refracted through the fiber body 13. In practice, a laser, e.g. a HeNe laser, is the preferred light source because of its wavelength stability. The following discussion is thus in terms of a laser light source, it being understood that other light sources having sufficient spatial coherence and monochromaticity can be used if desired.
As explained in the Watkins patents, in the far field, this reflected and refracted light interferes to form fringe pattern 19. For an optical waveguide fiber having a core and a cladding, the fringe pattern will in general be a function of the wavelength of the incident light and of the indices of refraction and the diameters of both the core and the cladding. However, as shown by Watkins, if the core/clad ratio is not too large and if the fringe pattern is examined at sufficiently large angles, e.g., above about .+-.50.degree. in FIG. 1 for core/clad ratios of less than about 0.5, the pattern will depend almost exclusively on the diameter and index of refraction of the cladding.
Accordingly, if the index of refraction (n) of the cladding is known, the outside diameter (d) of the fiber can be determined by analyzing the fringe pattern. Specifically, the diameter can be approximated with good precision by counting the number of full and partial fringes (N) between two angles (.theta..sub.a and .theta..sub.b) and then using the following equations to calculate d: EQU E(.theta..sub.a)=sin (.theta..sub.a /2)+[n.sup.2 +1-2n cos (.theta..sub.a /2)].sup.1/2 ( 1) EQU E(.theta..sub.b)=sin (.theta..sub.b /2)+[n.sup.2 +1-2n cos (.theta..sub.b /2)].sup.1/2 ( 2) EQU d=N.lambda./[E(.theta..sub.b)-E(.theta..sub.a)] (3)
where .lambda. is the wavelength of the laser light used to illuminate the fiber. Note that in equation 3, there is a direct relationship between diameter and fringe count. In practice, given an invariant clad index and an invariant wavelength, one can calibrate the system with an empirical constant, which when multiplied by the number of fringes gives, the diameter.
Using the fringe counting technique, root mean square (RMS) precisions on the order of 0.2 microns have been achieved for detectors having angular extents of approximately 80.degree.. (The term "precision" is used herein in the sense of a 1 .sigma. repeatability of the fiber diameter measurement, e.g., a diameter measurement is precise to the 0.2 micron level if repeated measurements of a fiber of constant diameter have a scatter whose standard deviation .sigma. is less than or equal to 0.2 microns.) For a fiber having a diameter of around 125 microns, this corresponds to a percentage error of less than a two tenths of a percent. Although clearly quite precise, even higher levels of precision are needed to meet the demand for ever improved optical waveguide fibers.
For example, as optical fiber telecommunications are installed closer to the subscriber, the number of fiber-to-fiber connections and splices required in the field increases rapidly. These connections and splices need to be both easy to make and must have a very low loss. Generally, for single-mode fiber, good diameter control is crucial to meeting these two requirements. Specifically, one desires a one .sigma. variation among fiber diameters of approximately 0.2 microns or less. Given this target, one would prefer that the system which measures and/or controls the fiber diameter have a precision of 0.02 microns or less. This precision is a factor of ten beyond the capability of fringe counting techniques used in the past.
Refinements of the basic Watkins technique can be found in various patents including Frazee, Jr. et al. U.S. Pat. No. 4,027,977 (determination of core/clad ratio by detecting the angle of maximum modulation of the fringe pattern); Murphy et al. U.S. Pat. No. 4,280,827 (use of delay circuits and comparators to analyze fringe patterns); and Smithgall, Sr. U.S. Pat. No. 4,046,536 (analysis of fringe counts in the presence of "dropouts" resulting from faults in the fiber). None of these references provide techniques for achieving the desired precision level of 0.02 microns and below.
The use of fast Fourier transforms (FFTs) to analyze fringe patterns is discussed in an article entitled "Measurement of Optical Fiber Diameter Using the Fast Fourier Transform" by Mustafa Abushagur and Nicholas George, Applied Optics, Jun. 15, 1980, vol. 19, no. 12, 2031-2033. Abushagur and George report a consistency of 0.6% in measurements obtained using their FFT method. Significantly, this error level is greater than that achieved using the conventional fringe counting techniques (see above).
Other optical techniques for measuring fiber properties, including fiber diameters, can be found in Bailey et al. U.S. Pat. No. 4,924,087 (detection of fiber defects using light scattered out of the plane of the basic diffraction pattern); Douklias U.S. Pat. No. 4,501,492 (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); Eichenbaum U.S. Pat. No. 4,363,827 (detection of "caustic" surfaces in the pattern of scattered light produced by a coated optical fiber in order to control the coating process); Maillard et al. U.S. Pat. No. 4,541,856 (use of "diffused" light to detect bubbles, blisters, and solid particles in a stream of molten glass); Millet et al. U.S. Pat. No. 4,847,509 (use of two perpendicular optical systems to measure fiber diameter in which each system forms a blurred image of the fiber on a strip of photodetectors); Presby U.S. Pat. No. 4,307,296 (measurement of core diameter by inducing fluorescence of an index-modifying dopant in the core); and Young, II U.S. Pat. No. 4,136,961 (detection of defects in glass blanks by rotating the blank through a thin beam of light).
The use of near-field resonant backscattered light to determine fiber diameters and ellipticity is discussed in an article entitled "Outer Diameter Measurement of Low Birefringence Optical Fibers by a New Resonant Backscatter Technique," by A. Ashkin, J.M. Dziedzic, and R.H. Stolen, Applied Optics, Jul. 1, 1981, vol. 20, no. 13, 2299-2303.
In accordance with this technique, the fiber is illuminated with light from a tunable laser. As the wavelength of the light produced by the laser is varied, resonances appear in the backscatter light. The authors predict that these resonances can be used to measure relative changes in the diameter or shape of the fiber with precisions on the order of 0.01 to 0.001 microns.
As acknowledged in their article, Ashkin et al. were only able to use their technique to make relative fiber diameter measurements, not absolute measurements. Also, although they describe real-time fiber measurements as a "possible extension" of their technique, the work reported in their article was all conducted under controlled laboratory conditions. Thus, as with the rest of the prior art, Ashkin et al. do not achieve absolute diameter measurements having precision levels of 0.02 microns and below under real operating conditions.